1. Field of the Invention
This invention relates to instruments for measurement of electron intensity distributions and more particularly to the measurement of illumination uniformity in electron beam projection systems.
2. Description of Related Art
Prior art pinhole electron detectors comprise a heavy metal barrier with an aperture (pinhole) through which electrons pass to a detector. A typical detector comprises a scintillator which emits light in response to electron impact and a photomultiplier tube to detect the light and electronically amplify the signal.
An image of an electron intensity distribution can be acquired by scanning the distribution across a pinhole detector. The spatial resolution of the resulting image or data is limited by the size of the pinhole and the extent to which the pinhole edges provide an abrupt transition between electron transmission and electron blockage. Electron stopping power increases with atomic number. For energetic electron beams (75 keV), a thickness of 5-10 micrometers of high atomic number material, e.g. Au, is sufficient to stop the electrons, whereas 100-125 micrometers of a low atomic number material, such as carbon, would be required. Heretofore, no techniques have existed to fabricate pinholes of several micrometers diameter in materials as thick as 100-125 micrometers, so pinholes were fabricated in heavy metals such as gold, platinum, and molybdenum.
U.S. Pat. No. 4,578,587 of Behringer et al. for xe2x80x9cError-Corrected Corpuscular Beam Lithographyxe2x80x9d shows a single hole diaphragm for accurately determining the shape of a thin tightly stretched foil mask by means of selected mask positions. A diaphragm having a hole of typically 0.5 xcexcm (diameter or lateral length) is provided on the table outside of the support area for the wafer. Below the diaphragm, the single hole diaphragm and a scintillator are positioned. In the scintillator impinging electrons from the electron beam produce light flashes. The light flashes are fed to the photomultiplier. In a multihole arrangement the diaphragm used is a gold diaphragm and the holes, once again, are 0.5 xcexcm in diameter or lateral length.
See U.S. Pat. No. 5,180,919 of Oae et al. for xe2x80x9cElectron Beam Exposure System Having the Capability of Checking the Pattern of an Electron Mask Used for Shaping an Electron Beamxe2x80x9d which passes a beam through a pattern of apertures in a block mask and through an aperture in a blanking aperture plate.
In the past, use has been made of conventional pinhole detectors for providing the required measurements. Use of such equipment to provide the signal required for a new Electron Beam Projection System (EBPS) system has been tried, but in the current state of the art the results have been unsatisfactory. These pinholes all cause significant backscattering of electrons which produces artifacts.
The artifacts preclude the measurements of electron intensity distribution with sufficient accuracy.
An advantage of this invention is that a pinhole plate formed of a material of low atomic number has low backscatter and reduces artifacts.
In accordance with this invention, an instrument for measurement of illumination intensity distribution comprises as follows:
a pinhole plate comprising a block of low atomic number material with a high aspect ratio pinhole therethrough,
means for detecting electrons passing through the pinhole, and
means for moving, scanning, the pinhole with respect to a fixed electron intensity distribution or scanning the electron intensity distribution with respect to pinhole when held in a fixed position.
In accordance with this invention, an instrument for measurement of illumination intensity distribution of an Electron Beam Projection System (EBPS) comprises as follows:
a reticle with a clear subfield,
a pinhole plate comprising a block of low atomic number material with a high aspect ratio pinhole therethrough, said pinhole plate being positioned at the image plane of said EBPS,
means for detecting electrons passing through said pinhole, and
means for moving, scanning, the pinhole with respect to a fixed electron intensity distribution or scanning the electron intensity distribution with respect to pinhole when held in a fixed position.